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Re: [Qemu-block] [Qemu-devel] [PATCH v4 02/11] block: Filtered children

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH v4 02/11] block: Filtered children access functions
Date: Tue, 7 May 2019 09:32:34 +0000

24.04.2019 19:36, Max Reitz wrote:
> On 19.04.19 12:23, Vladimir Sementsov-Ogievskiy wrote:
>> 17.04.2019 19:22, Max Reitz wrote:
>>> On 16.04.19 12:02, Vladimir Sementsov-Ogievskiy wrote:
>>>> 10.04.2019 23:20, Max Reitz wrote:
>>>>> What bs->file and bs->backing mean depends on the node.  For filter
>>>>> nodes, both signify a node that will eventually receive all R/W
>>>>> accesses.  For format nodes, bs->file contains metadata and data, and
>>>>> bs->backing will not receive writes -- instead, writes are COWed to
>>>>> bs->file.  Usually.
>>>>> In any case, it is not trivial to guess what a child means exactly with
>>>>> our currently limited form of expression.  It is better to introduce
>>>>> some functions that actually guarantee a meaning:
>>>>> - bdrv_filtered_cow_child() will return the child that receives requests
>>>>>      filtered through COW.  That is, reads may or may not be forwarded
>>>>>      (depending on the overlay's allocation status), but writes never go 
>>>>> to
>>>>>      this child.
>>>>> - bdrv_filtered_rw_child() will return the child that receives requests
>>>>>      filtered through some very plain process.  Reads and writes issued to
>>>>>      the parent will go to the child as well (although timing, etc. may be
>>>>>      modified).
>>>>> - All drivers but quorum (but quorum is pretty opaque to the general
>>>>>      block layer anyway) always only have one of these children: All read
>>>>>      requests must be served from the filtered_rw_child (if it exists), so
>>>>>      if there was a filtered_cow_child in addition, it would not receive
>>>>>      any requests at all.
>>>>>      (The closest here is mirror, where all requests are passed on to the
>>>>>      source, but with write-blocking, write requests are "COWed" to the
>>>>>      target.  But that just means that the target is a special child that
>>>>>      cannot be introspected by the generic block layer functions, and that
>>>>>      source is a filtered_rw_child.)
>>>>>      Therefore, we can also add bdrv_filtered_child() which returns that
>>>>>      one child (or NULL, if there is no filtered child).
>>>>> Also, many places in the current block layer should be skipping filters
>>>>> (all filters or just the ones added implicitly, it depends) when going
>>>>> through a block node chain.  They do not do that currently, but this
>>>>> patch makes them.
>>>>> One example for this is qemu-img map, which should skip filters and only
>>>>> look at the COW elements in the graph.  The change to iotest 204's
>>>>> reference output shows how using blkdebug on top of a COW node used to
>>>>> make qemu-img map disregard the rest of the backing chain, but with this
>>>>> patch, the allocation in the base image is reported correctly.
>>>>> Furthermore, a note should be made that sometimes we do want to access
>>>>> bs->backing directly.  This is whenever the operation in question is not
>>>>> about accessing the COW child, but the "backing" child, be it COW or
>>>>> not.  This is the case in functions such as bdrv_open_backing_file() or
>>>>> whenever we have to deal with the special behavior of @backing as a
>>>>> blockdev option, which is that it does not default to null like all
>>>>> other child references do.
>>>>> Finally, the query functions (query-block and query-named-block-nodes)
>>>>> are modified to return any filtered child under "backing", not just
>>>>> bs->backing or COW children.  This is so that filters do not interrupt
>>>>> the reported backing chain.  This changes the output of iotest 184, as
>>>>> the throttled node now appears as a backing child.
>>>>> Signed-off-by: Max Reitz <address@hidden>
>>>>> ---
>>>>>     qapi/block-core.json           |   4 +
>>>>>     include/block/block.h          |   1 +
>>>>>     include/block/block_int.h      |  40 +++++--
>>>>>     block.c                        | 210 +++++++++++++++++++++++++++------
>>>>>     block/backup.c                 |   8 +-
>>>>>     block/block-backend.c          |  16 ++-
>>>>>     block/commit.c                 |  33 +++---
>>>>>     block/io.c                     |  45 ++++---
>>>>>     block/mirror.c                 |  21 ++--
>>>>>     block/qapi.c                   |  30 +++--
>>>>>     block/stream.c                 |  13 +-
>>>>>     blockdev.c                     |  88 +++++++++++---
>>>>>     migration/block-dirty-bitmap.c |   4 +-
>>>>>     nbd/server.c                   |   6 +-
>>>>>     qemu-img.c                     |  29 ++---
>>>>>     tests/qemu-iotests/184.out     |   7 +-
>>>>>     tests/qemu-iotests/204.out     |   1 +
>>>>>     17 files changed, 411 insertions(+), 145 deletions(-)
>>>> really huge... didn't you consider conversion file-by-file?
>>> Frankly, no, I just didn’t consider it.
>>> Hm.  I don’t know, 30-patch series always look so frightening.
>>>>> diff --git a/block.c b/block.c
>>>>> index 16615bc876..e8f6febda0 100644
>>>>> --- a/block.c
>>>>> +++ b/block.c
>>>> [..]
>>>>> @@ -3467,14 +3469,17 @@ static int 
>>>>> bdrv_reopen_parse_backing(BDRVReopenState *reopen_state,
>>>>>         /*
>>>>>          * Find the "actual" backing file by skipping all links that point
>>>>>          * to an implicit node, if any (e.g. a commit filter node).
>>>>> +     * We cannot use any of the bdrv_skip_*() functions here because
>>>>> +     * those return the first explicit node, while we are looking for
>>>>> +     * its overlay here.
>>>>>          */
>>>>>         overlay_bs = bs;
>>>>> -    while (backing_bs(overlay_bs) && backing_bs(overlay_bs)->implicit) {
>>>>> -        overlay_bs = backing_bs(overlay_bs);
>>>>> +    while (overlay_bs->backing && 
>>>>> bdrv_filtered_bs(overlay_bs)->implicit) {
>>>> So, you don't want to skip implicit filters with 'file' child? Then, why 
>>>> not to use
>>>> child_bs(overlay_bs->backing), like in following if condition?
>>> I think it was an artifact of writing the patch.  I started with
>>> bdrv_filtered_bs() and then realized this depends on ->backing,
>>> actually.  There was no functional difference so I left it as it was.
>>> But you’re right, it is more clear to use child_bs(overlay_bs->backing)
>>> isntead.
>>>> Could we instead make backing-based filters equal to file-based, to make 
>>>> it possible
>>>> to use file-based filters in backing-chain related scenarios (like 
>>>> upcoming copy-on-read
>>>> filter for stream)? So, to expand backing-chain concept to include filters 
>>>> with file child?
>>> If I understand you correctly, that’s basically the purpose of this
>>> series and especially this patch here.  As far as it is possible and
>>> reasonable, I want filters that use bs->backing and bs->file behave the
>>> same.
>>> However, there are cases where this is not possible and
>>> bdrv_reopen_parse_backing() is one such case.  bs->backing and bs->file
>>> correspond to QAPI names, namely 'backing' and 'file'.  If that
>>> distinction was already visible to the user, we cannot change it now.
>>> We definitely cannot make file-based filters use bs->backing now because
>>> you can create them over QAPI and they use 'file' as their child name.
>>> Can we make backing-based filters use bs->file?  Seems more likely,
>>> because all of them are implicit nodes, so the user usually doesn’t see
>>> them.  But usually isn’t always; they do become user-visible once the
>>> user specifies a node-name for mirror or commit.
>>> I found it more reasonable to introduce new functions that explicitly
>>> express what kind of child they expect and then apply them everywhere as
>>> I saw fit, instead of making the mirror/commit filter drivers use
>>> bs->file and hope it works; not least because I’d still have to go
>>> through the whole block layer and check every instance of bs->backing to
>>> see whether it really needs bs->backing or whether it should use either
>>> of bs->backing or bs->file.
>>>>> +        overlay_bs = bdrv_filtered_bs(overlay_bs);
>>>>>         }
>>>>>         /* If we want to replace the backing file we need some extra 
>>>>> checks */
>>>>> -    if (new_backing_bs != backing_bs(overlay_bs)) {
>>>>> +    if (new_backing_bs != child_bs(overlay_bs->backing)) { >           
>>>>> /* Check for implicit nodes between bs and its backing file */
>>>>>             if (bs != overlay_bs) {
>>>>>                 error_setg(errp, "Cannot change backing link if '%s' has "
>>>> [..]
>>>>> @@ -4203,8 +4208,8 @@ int bdrv_change_backing_file(BlockDriverState *bs,
>>>>>     BlockDriverState *bdrv_find_overlay(BlockDriverState *active,
>>>>>                                         BlockDriverState *bs)
>>>>>     {
>>>>> -    while (active && bs != backing_bs(active)) {
>>>>> -        active = backing_bs(active);
>>>>> +    while (active && bs != bdrv_filtered_bs(active)) {
>>>> hmm and here you actually support backing-chain with file-child-based 
>>>> filters in it..
>>> Yes, because this is not about the QAPI 'backing' link.  This function
>>> should continue to work even if there are filters in the backing chain.
>> this is a generic function to find overlay in backing chain and it may be 
>> used from different places,
>> for example it is used in Andrey's series about filter for block-stream.
> Well, all places that use it accept backing chains with filters inside
> of them.
>> It is used from qmp_block_commit, isn't it about QAPI?
> By "QAPI 'backing' link" I mean the user-visible block graph.  Hm.  I
> wrote in my other mail that you could use query-named-block-nodes to see
> that graph; apparently you can’t.  So besides x-debug-query-block-graph,
> we still don’t have any facility to query the block graph?  I don’t know
> what to say.
> Anyway, you can still construct the graph with blockdev-add, so it is
> user-visible.  And in that block graph, there is a 'backing' link, and
> there is a 'file' link -- this is what I mean with "QAPI link".
> We have commands that are abstract and don’t work on specific graph
> links.  For instance, block-commit commits across a backing chain, so it
> doesn’t matter whether the graph link is called 'backing' or whatever,
> what is important is that it’s a COW link.  But we should also ignore
> filters on the way, so this patch makes block-commit and others use
> those more abstract child access functions.
> But whenever it is about exactly the "file" or the "backing" link, we
> have to use bs->file and bs->backing, respectively.  That's just how it
> currently is.
>>>>> +        active = bdrv_filtered_bs(active);
>>>>>         }
>>>>>         return active;
>>>>> @@ -4226,11 +4231,11 @@ bool 
>>>>> bdrv_is_backing_chain_frozen(BlockDriverState *bs, BlockDriverState *base,
>>>>>     {
>>>>>         BlockDriverState *i;
>>>>> -    for (i = bs; i != base; i = backing_bs(i)) {
>>>>> +    for (i = bs; i != base; i = child_bs(i->backing)) {
>>>> and here don't..
>>> Yes, because this function is about the QAPI 'backing' link.
>> And this again a generic thing, that may be used in same places as 
>> bdrv_find_overlay,
> But it isn’t.
>> and it is used in series about block-stream filter too. So, for further 
>> developments
>> we'll have to keep in mind all these differences between generic block layer 
>> functions,
>> which supports .file children inside backing chain and which are not...
> I was wrong about bdrv_is_backing_chain_frozen(), if that helps (as I
> wrote in my other (previous) mail).
> But for example bdrv_set_backing_hd() always has to use bs->backing,
> because that’s what it’s about (and I do change its descriptive comment
> to reflect that, so you don’t need to keep it in mind).  Same for
> bdrv_open_backing_file().
> Hm, what other cases are there...
> bdrv_reopen_parse_backing(): Fundamentally, this too is about the
> user-visible "backing" link (as specified through x-blockdev-reopen).
> But the loop it contains is more difficult to translate than I had
> thought.  At some point, there needs to be a bs->backing link, because
> that is what this function is about, but it should also skip all
> implicit filters in the way, I think.  So e.g. this should be recognized:
> bs  ---backing-->  COR ---file-->  base
> @overlay_bs should be COR, I think...?  I mean, as long as COR is an
> implicit node.  So the loop really should use bdrv_filtered_bs()
> everywhere, and then the same afterwards.  I think that we should also
> ensure that @bs can support a ->backing child, but how would I check
> that?  Maybe it’s safe to just omit such a check...
> But then another issue comes in: The link to replace (in the above case
> from "COR" to "base") is no longer necessarily a backing link.  So
> bdrv_reopen_commit() has to be capable of replacing both bs->backing and
> bs->file.
> Actually, how does bdrv_reopen_commit() handle implicit nodes at all?
> bdrv_reopen_parse_backing() just sets reopen_state->replace_backing_bs
> and ->new_backing_bs.  It doesn’t communicate anything about overlay_bs.
>   bdrv_reopen_commit() then asserts that !bs->backing->bs->implicit and
> replaces bs->backing.  So it seems to just fail on the implicit nodes
> that bdrv_reopen_parse_backing() took care to skip...
> OK, what else...  bdrv_reopen_prepare() checks
> reopen_state->bs->backing, which I claim is correct because while there
> may be implicit filters in the chain, the first link has to be a
> ->backing link.

[sorry for a long delay]
Are you working on next version or waiting for more reviews?

Why first link should be backing? We want to skip all implicit filters, 
file-child-based in following call to bdrv_reopen_parse_backing(). So, don't we
want something like bdrv_backing_chain_next() here? But then a question, could
reopen_state->bs be filter itself...

> bdrv_backing_overridden() has to query bs->backing because this function
> is used when it is about a specific characteristic of the backing link:
> There is a non-null default (given by the image header), so if the
> current bs->backing matches this default, you do not have to specify the
> backing filename in either blockdev-add or a filename.  Same in
> bdrv_refresh_filename().
> I hope that was all...?
> Max

Best regards,

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